1. Field of the Invention
The present invention relates to an optical recording medium comprising a substrate formed therein with prepits between pregrooves, and an optical absorption layer and an optical reflection layer which are formed on the substrate, and in particular, to a high density optical recording medium which can reduce the error rate of address signals, and the jitter and the block error rate of recording signals.
2. Related Art
These years, that is, in the highly information-oriented era, optical recording media which can deal with several various information become noticeable. These optical recording media are classified into a read-only recording medium on which no data can be recorded, and a write-once type optical recording medium on which date can be only once written, and a rewritable recording medium on which data can be rewritten many times. Of these optical recording media, the write-once type optical recording medium is suitably used as a data storage medium for official documents and important documents since data recorded once thereon cannot be altered for defrauding or the like. As such a write-once type optical recording medium, there has been know, for example, a CD-R (compact disc-recordable).
This CD-R which is a write-once type CD has an optical absorption layer and an optical reflection layer formed on a substrate and made of organic pigment, and which can produce output signals which are in conformity with a CD format during reproduction. The substrate of this CD-R is spirally or concentrially formed thereon with pregrooves for tracking. In order to record data on the CD-R, a laser beam is irradiated on the substrate along the pregrooves so as to decompose or alter the optical absorption layer. Thus, recording marks are formed in the recording beam irradiated part of the optical absorption layer within pregrooves. When a reproduction beam is irradiated along the pregrooves, data can be read since a difference in optical reflectivity is caused between the parts on which the recording marks are formed, and parts on which no recording marks are formed, and since the difference in the optical reflectivity can be used for reading the data. The laser beam used for recording and reproducing to and from the CD-R usually has a wavelength in a range from 770 to 830 nm.
These years, it has been demanded that the recording capacity of a recording medium is further increased. In order to increase the recording capacity, there has been proposed such a method that the wavelength of the laser beam used for recording and reproducing is shortened so as to decrease the diameter of a beam spot in order to enhance the recording density. As to a high density recording medium in association with this method, there have been proposed a DVD (digital versatile disc) and a DVD-R (digital versatile disc-recordable) (refer to, for example, Electronic Technology, August, 1996, on page 101). A laser beam used for recording and reproduction of this DVD-R has a wavelength in the range from 630 to 660 nm.
Pregrooves for tracking are spirally and concentrically formed on a substrate of a DVD-R, as shown in FIG. 5, and prepits 41 for address signals are formed at predetermined pitches between adjacent pregrooves 42, 42xe2x80x2 (in lands therebetween). Further, these prepets are provided in combination with their inner peripheral side pregrove 42. Track pitches of the pregrooves in the DVD-R are in a range from 0.7 to 0.8 xcexcm, which is extremely dense in comparsion with track pitches of the pregrooves in the CD-R, which are 1.6 xcexcm.
Accordingly, as to the DVD-R, the width of the pregrooves becomes smaller than the diameter of the beam spot for recording and reproduction, and the lands laid on both sides of a pregroove are in part included within the beam spot with which the pegroove is scanned for recording and reproduction. In this case, if the prepits are present in either an inner peripheral side land or an outer peripheral side land in the vicinity of recording pits formed in the pregroove, a part of the prepits is included within the spot for recording and reproduction when the recording pits are reproduced. Thus, a difference in obtained reproduction intensity is caused between a recording pit in the vicinity of a prepit, and a recording pit which is far from a prepit even though the recording pits have one and the same pit length. Accordingly, these pits are reproduced as recording pits having different pit lengths. As a result, there has been caused such a problem that jitter and block error rate become high. Further, in order to solve this problem, there has been proposed such a method that the size of these prepits formed in the land between the pregrooves is decreased. However, in this method, the intentity of prepit signals becomes smaller, and accordingly, there has been caused such a problem that the error rate of address signals becomes high.
The present invention is devised in order to solve the above-mentioned problems, and accordingly, an object of the present invention is to provide a high dense optical recording medium in which prepits formed (in lands) between pregrooves can be precisely reproduced so as to reduce an error rate of address signals, and recording pits formed in the vicinity of the prepits can be precisely reproduced, thereby it is possible to reduce jitter and a block error rate of recording signals.
To the end, according to an object of the present invention, there is provided an optical recording medium comprising a substrate formed therein prepits in (lands) between pregrooves, the pregrooves and the prepits being set apart from each other by edge parts therebetween, and an optical absorption layer and an optical reflection layer which are formed on the substrate, the prepits are preferably formed so as to satisfy the following inequal equation (1):
0 less than bxe2x89xa6a less than Dpxe2x89xa6Dgxe2x80x83xe2x80x83(1)
where Dg is a maximum depth of the pregrooves, Dp is a maximum depth of the prepits, a is a depth of an edge parts on the inner peripheral side of the prepits,and b is a depth of the edge part on the outer peripheral side of the prepits.
FIGS. 2 and 3 show a specific example of the shape of the prepits formed (in lands) between the pregrooves, which can satisfy the above-mentioned condition, and are a perspective view and an enlarged partly sectional view, respectively, illustrating a sectioned structure obtained by cutting the substrate by a plane that is orthogonal to the longitudinal direction of the pregrooves (pregroove direction) and including prepits. It is noted that the pregrooves and the prepits are set apart from each other by eges parts 23, 23xe2x80x2 therebetween.
Referring to FIG. 3, the maximum depth Dg of the pregrooves 22, the maximum depth Dp of the prepits 21, the depth a of the edge parts 23 on the inner peripheral side of the prepits 22, and the depth b of edge parts 23xe2x80x2 on the outer peripheral side of the prepits 21 satisfy the above-mentioned unequal equation (1). As shown in FIG. 5, in such a case that no edge parts are formed in the prepits formed (in lands) between grooves 42, 42xe2x80x2, as in the conventional one, so that the pregrooves and the prepits are continuously formed, a part of a prepit which is adjacent to the pregroove is recognized as a recording pit. On the contrary, according to the present invention, since the edge part between the pregroove and the prepit is formed so that it is higher than the bottom surface of the prepit, and in more detail, the depth of the edge part between the prepit and the pregroove is set to be less than 95% of the maximum depth Dp of the prepit, it is possible to prevent the prepit from being recognized as a recording pit even though a part of the prepit is contained in the reproduction spot. Thus, recording pits can be appropriately reproduced, irrespective of the presence of a prepit adjacent to a recording pit. Accordingly, the optical recording medium according to the present invention can reduce jitter and a block error rate of recording signals.
Further, since a signal of a prepit 21 on the outer periheral side, which is in a pair with a pregrooves 22 can be appropriately reproduced with a sufficient intensity, the depth a of the edge parts 23 on the inner periheral side of the prepits 21 is set to be greater than 20% of the maximum depth Dp of the prepits 21 while the maximum depth Dp of the prepits 21 is set to be greater than 50% of the maximum depth Dg of the pregrooves 22. Further, the pripits 21 is adjacent also to the pregroove 22xe2x80x2 on the outer peripheral side of the prepits 21, and the depth b of the edge parts 23xe2x80x2 on the outer peripheral side of the prepits, is preferably set to be equal to or less than the depth a of the edge parts 23 on the inner peripheral side of the prepits. Accordingly, the optical recording medium according to the present invention can reduce an error rate of the address signals.
Further, according to the present invention, it is preferable to form the prepits so as to satisfy the following unequal equation (2):
0 less than Bxe2x89xa6Axe2x80x83xe2x80x83(2)
where A is the length of the edge parts 23 on the inner peripheral side of the prepits 21 as shown in FIG. 4, in the pregroove direction, and B is the longth of the edge parts 23xe2x80x2 on the outer peripheral side of the prepits 21 in the pregroove direction.
The shape of the prepits 21 according to the present invention is optional if the prepits are adjacent to pregrooves 22, 22xe2x80x2 on both sides, and the lengths A, B of both edges of the prepits can satisfiy the above-mentioned unequal equation (2).
The substrate of the optical recording medium according to the present invention, may be made of arbitrary materials having optical transparency, which is transparent resin such as polycarbonate resin or polymethacrylate methyl resin. Although the substrate can be manufactured by injection-molding the above-mentioned resin material with the use of, for example, an injection molding machine mounted thereon with a stamper formed therein with a preformat pattern, the present invention should not be limited to this manufacturing method, but it may be manufactured with the use of a 2P (Photo-polymerizatoin) method. The intervals of the pregrooves for tracking formed at least one surface of the substrate is preferably set in a range from 0.6 to 0.9 xcexcm in order to increase the recording density. Further, the depth of the pregrooves is preferably set in a range from 100 to 250 nm in order to obtain reproducing signals with the use of an optical interference effect.
The optical absorbing layer of the optical recording medium according to the present invention is made of azo group organic pigment, cyanin groupd organic pigment or pigment obtained by mixing these pigments. The optical absorbing layer may be formed by spin-coat, vacuum evaporation or the like, and among them, a spin coat process using a solvent is more preferable. As to the solvent using during the formation of the optical absorbing layer, ethylcellosolve, methylcellosolve, methanol, tetrafluoro propanol or the like may be used. The thickness of the optical layer to be formed is preferably in a range from 20 to 200 nm, Further, the optical layer may be added therein with singlet oxygen quencher in order to prevent optical deterioration of the cyanin pigment or the azo pigment.
In the optical recording medium according to the present invention, the optical reflecting layer may be made of metal such as Au, Ag, Cu, Al or alloy containing, as a main component, one of these metals, and may be formed by vacuum evaporation, sputtering, ion plating or the like. The thickness of the optical reflecting layer is preferably in a range from 0.02 to 0.5 mm.
In the optical recording medium according to the present invention, a protecting layer may be further formed on the optical reflecting layer. The protecting layer may be made of ultraviolet curing resin, epoxy resin, acrylic resin, silicon resin, urethane resin or the like. The thickness of the protecting layer is preferably in a range from 0.1 to 100 xcexcm, and in more preferably in a range from 1 to 20 xcexcm.
The present invention will be detailed in the form of preferred embodiments to which the present invention should be limited, with reference to the accompanying drawings in which: